In Offenlegungsschrift DE No. 32 37 079 A1 a process is described by which it is possible to convert methane in the presence of oxygen or an oxygen containing gas and in the presence of suitable catalysts at temperatures between 500.degree. and 900.degree. C. to ethane and ethylene according to the following chemical equation: ##STR1## Besides the C.sub.2+ hydrocarbons also undesired products i.e. CO and CO.sub.2 can be formed.
By applying higher reaction temperatures the yield as well as the selectivity to C.sub.2+ hydrocarbons, particularly to ethylene is increased. The catalysts used contain as catalytically active compounds the oxides of lead, antimonium, tin, bismut, cadmium, thallium, indium or their mixtures. The catalytically active compound may be used alone or together with one or several compounds being stable at higher temperatures and which are called support or support material. Such supports may be alumina, silica, titania, aluminum silicate or similar materials. Also some temperature resistant carbides like silicium carbide can be used. The use of a support is necessary to avoid sintering of the catalytically active compound when exposed to high temperatures in order to sustain a large surface area.
The oxides of several metals which can change their valency states under reaction conditions can directly be used as oxygen transferring materials for the oxidative coupling; they have, however, to be reoxidized. Such metals are antimonium (U.S. Pat. No. 4,443,644), germanium (U.S. Pat. No. 4,443,645), bismut (U.S. Pat. No. 4,443,646), lead (U.S. Pat. No. 4,443,647), indium (U.S. Pat. No. 4,443,648), manganese (U.S. Pat. No. 4,443,649) and tin (U.S. Pat. No. 4,444,984). The performance of such oxygen transferring agents which are no catalysts in the stricter sense of the words, can be improved by the addition of phosphates, alkali oxides (U.S. Pat. No. 4,499,322), alkaline earth oxides (U.S. Pat. No. 4,495,374), CeO.sub.2 (U.S. Pat. No. 4,499,324), RuO.sub.2 (U.S. Pat. No. 4,489,215) and Pr.sub.6 O.sub.11 (U.S. Pat. No. 4,499,323).
Maximum selectivities for C.sub.2+ hydrocarbons (n.gtoreq.2) of approx. 50 to 70% were obtained in a continuous process for the oxidative coupling of methane when using catalysts containing PbO (comp. a) German patent application No. P 34 06 751.5 and (b) W. Hinsen, W. Bytyn, M. Baerns in Proceed. 8th Intern. Congr. Catal. 1984 Vol. III, p. 581, Verlag Chemie 1984).
There exists a certain disadvantage of such catalysts containing lead oxide or some of the other metal oxides mentioned above since their vapour pressures might not be negligible in some cases at the high reaction temperatures (&gt;500.degree. C.) required. High temperatures may result in a loss of catalytically active compound and leads eventually to deactivation. Furthermore, the deposition of lead or lead oxide or the other metals or metal oxides respectively in the cold parts of the production plant causes severe problems detrimental to smooth operation.